BackwardsIndex on a ref array fails to compile in procs

[ShadowElf37]

BackwardsIndex [^i] on a ref array will fail to compile inside procedures, iterators, funcs, and methods. It will compile if you dereference the array explicitly, but this is contrary to the behavior of normal indices, as well as backwards indices outside a proc.

Example

var a = new array[1, int]
a[] = [2,]

# WORKS
echo a[0]

# WORKS
echo a[^1]

# WORKS
proc proc1[S: static[int], T](arr: ref array[S, T]): int =
    return arr[][0]

# WORKS
proc proc2[S: static[int], T](arr: ref array[S, T]): int =
    return arr[0]

# WORKS
proc proc3[S: static[int], T](arr: ref array[S, T]): int =
    return arr[][^1]

# DOESNT COMPILE
proc proc4[S: static[int], T](arr: ref array[S, T]): int =
    return arr[^1]

Error

Error: type mismatch: got <ref array[0..0, int], BackwardsIndex>
but expected one of: 
proc `[]`(s: string; i: BackwardsIndex): char
  first type mismatch at position: 0
proc `[]`[I: Ordinal; T](a: T; i: I): T
  first type mismatch at position: 0
proc `[]`[Idx, T, U, V](a: array[Idx, T]; x: HSlice[U, V]): seq[T]
  first type mismatch at position: 0
proc `[]`[Idx, T](a: array[Idx, T]; i: BackwardsIndex): T
  first type mismatch at position: 0
proc `[]`[Idx, T](a: var array[Idx, T]; i: BackwardsIndex): var T
  first type mismatch at position: 0
proc `[]`[T, U, V](s: openArray[T]; x: HSlice[U, V]): seq[T]
  first type mismatch at position: 0
proc `[]`[T, U](s: string; x: HSlice[T, U]): string
  first type mismatch at position: 0
proc `[]`[T](s: openArray[T]; i: BackwardsIndex): T
  first type mismatch at position: 0
proc `[]`[T](s: var openArray[T]; i: BackwardsIndex): var T
  first type mismatch at position: 0
template `[]`(s: string; i: int): char
  first type mismatch at position: 0

expression: `[]`(arr, BackwardsIndex(1))

Nim Compiler Version 1.4.4 [Windows: amd64]
Compiled at 2021-02-23
Copyright (c) 2006-2020 by Andreas Rumpf

[ghost]

relevant piece of code in the compiler: https://github.com/nim-lang/Nim/blob/devel/compiler/semexprs.nim#L1475 (if I understand correctly)

[metagn]

relevant piece of code in the compiler

Also mentioned here: #22970 (comment)

The solution here wouldn't be in the compiler though, BackwardsIndex which is implemented in user code just isn't implemented for ref array, for which normal indexing happens to work